Braking Distance Calculator

Speed, km/h

Reaction time, s

About one second for an alert, sober driver.

Surface

Gradient, % (downhill negative)

Result

Stopping distance, m67.5
Covered while reacting, m25
Braking itself, m42.5
The same 20 km/h faster, m94
In car lengths15
Surfacedry asphalt

Reaction plus braking

Works out the total stopping distance from speed, driver reaction time, the grip of the surface and the gradient of the road.

Stopping distance has two parts and the first one is easy to forget. While the driver is deciding, the car carries on exactly as before: at 90 km/h, one second of thinking is 25 metres covered without the pedal being touched.

The second part grows with the square of speed. Going from 90 to 110 adds 22 percent to the speed but nearly 50 percent to the braking distance. That is why 'only a bit faster' in the rain turns into 'did not make it'.

What this calculator shows

The full stopping distance, split into reaction and braking
The same manoeuvre 20 km/h faster, for comparison
The distance in car lengths, which is easier to picture

What to keep in mind

Grip coefficients: dry asphalt 0.75, wet 0.45, gravel 0.5, packed snow 0.25, ice 0.12.
The figures assume working brakes and seasonal tyres; worn tread on a wet road easily costs a quarter of the grip.

FAQs

What reaction time should I use?

About one second for an alert, sober driver. Tiredness, a phone call or alcohol stretch it to one and a half or two, which on a motorway is another 25 to 30 metres.

Why does braking distance grow faster than speed?

Because brakes have to dissipate kinetic energy, and that scales with the square of speed. Double the speed and the braking distance quadruples.

How much worse is a wet road?

Grip drops from about 0.75 to 0.45, so braking distance grows by half again or more. Packed snow triples it and ice multiplies it roughly sixfold.

Does ABS shorten the stopping distance?

On asphalt, barely — its job is to keep the car steerable. On loose snow and gravel it can even lengthen the distance, because locked wheels would build a wedge in front of them.

Worked example

Ninety on dry asphalt

Input: 90 km/h, one second of reaction, dry asphalt

Output: 67.5 metres, of which 25 are pure reaction

Note: On wet asphalt the same speed needs 95.8 metres. Nearly thirty metres more — seven car lengths — and nothing about the driving changed at all.

Useful links

All tools in Cars

Braking Distance Calculator

Result

Related tools

Tyre Size Calculator

Speed from RPM Calculator

Tyre Pressure and Temperature Calculator

Useful links